The innate immune system is the first line of defense against microbial and viral infections. A failure to elicit the early innate immune response leads to systemic infections. A significant number of human viruses, including Influenza, Hepatitis C, Dengue, West Nile, Respiratory Syncytial, Reovirus, and Ebola are recognized by the innate immunity receptor RIG-I. The overall goal of this proposal is to understand how RIG-I (Retinoic Acid-inducible Gene-I), a cytoplasmic receptor discriminates normal, cellular from viral RNAs to stimulate a host response. A major goal of our collaborative research studies has been to understand the thermodynamic, kinetic, and structural mechanisms by which RIG-I recognizes atypical pathogen associated molecular pattern (PAMP) feature in RNAs. Our crystal structure of RIG-I bound to blunt-ended dsRNA and an ATP analog established a new paradigm for RIG-I activation. The goal of this proposal is to rigorously test the RIG-I activation model using carefully designed biochemical, kinetic, and structural studies. Our preliminary results demonstrate that RIG-I is regulated in multiple ways and RNA binding affinity is not the only criterion for PAMP selection. With a unique collection of purified protein, RNA reagents, we will employ complementary biochemical, biophysical, structural, and cell based approaches to 1) understand the basis of PAMP versus non-PAMP recognition of RNA by RIG-I; 2) characterize RIG-I ATPase activity and its role in RIG-I activation; and 3) understand the mechanism of RIG-I signaling. The outcomes are better understanding of self versus non-self recognition and RIG-I evasion mechanism, which can lead to the development of broad-spectrum antivirals, anti-inflammatory therapeutics and RNA-based gene silencing agents.